Steer by-wire is an advanced steering concept for vehicles, that eliminates the mechanical connection between the steering wheel and the front wheel. All components of this vehicle steering system communicate digitally rather than mechanically. The drivers input over the steering wheel is converted into digital signals. Therefore, a sensor measures the torque and the angle of the steering wheel.
U.S. Pat. No. 5,930,905 discloses a method and a device for measuring the angle of a rotatable body, in particular a body rotatable by more than 360°. The rotatable body cooperates with at least two further rotatable bodies, for example gear wheels, where angular positions are determined with the aid of two sensors. The angular position of the rotatable body is determined from the detected angular positions by a modified nonius calculation so that unequivocal statements can be made, it is necessary for all three rotatable bodies or gear wheels to have defined predetermined numbers of teeth. The method and the device can be used for determining the steering wheel angle of a motor vehicle.
German Patent 10 041 095 is related to a device for measuring an angle and/or a torque on a rotatable body. The angle of rotation is detected by means of magnetic or optical sensors. In a preferred embodiment there are two devices, each provided with two optically readable code traces. The two code traces of each device are embodied in the same way and are arranged in such a way that said devices are offset against each other so that allocated sensors output a digital signal. The angle of rotation is calculated from the offset of the two digital signals. In another embodiment, a torsional element having a known torsional stiffness is arranged between the two devices. A torque that is transmitted by the rotatable body can thus be calculated from the difference angle of the two devices. This device is preferably used in the steering column shaft of a motor vehicle.
WO 00/28285 concerns an optical sensor for determining the position of a moveable surface having patterned regions of high and low reflectivity to EMR (electromagnetic radiation). The sensor comprises an ASIC (application specific integrated circuit), at least one lens and at least one EMR source. The ASIC comprises at least one array of EMR sensitive detectors and processing means. The EMR source facilitates illumination of the surface and the at least one lens facilitates the focusing of reflected EMR from the surface and generates an image on the at least one array of EMR sensitive detectors corresponding to the pattern on the surface. The ASIC, the at least one lens, and the at least one EMR source are all enclosed in a single housing providing accurate optical alignment of these elements and integrated as a single replaceable module. The processing means of the ASIC facilitates processing of the image to determine the position of the pattern on the surface.
According to the prior art, the pattern of the surface is formed, for example, by symmetrical turning marks and by laser marks thereon. The axis of symmetry of each symmetrical turning mark is positioned perpendicularly to the patterned surface. These marks may for example have a cross section in the form of a circular arc. The objective of the symmetrical turning marks is to reflect EMR coming from the EMR source into the image system containing at least one lens. As a result, the EMR reflected by the symmetrical turning marks produces a bright area on the array of EMR sensitive detectors. The laser marks consist of a flat surface on the patterned surface, designed for reflecting the EMR out of the image system. Consequently, the laser marks produce dark areas on the array of EMR sensitive detectors.
The resulting pattern of bright and dark areas can only be detected correctly, if the illumination efficiency is good and the contrast between dark and bright areas is high. These two parameters depend on the illumination angle between the EMR source and the patterned surface.
According to the prior art, high illumination efficiency is coupled with a low contrast and vice versa. As a result, the chosen illumination angle represents a compromise between optimizing the illumination efficiency on one side and the contrast on the other side.